Quantum Feedback Networks: Hamiltonian Formulation

TL;DR

This paper develops a Hamiltonian framework for quantum feedback networks, enabling a systematic and unified approach to modeling interconnected quantum systems with and without time delays, including feedback via beam splitters.

Contribution

It introduces a Hamiltonian formulation for quantum networks that generalizes previous models, incorporating component details, interactions, and feedback mechanisms within a unified system-theoretic approach.

Findings

Hamiltonian is symmetric and potentially self-adjoint.

Model reduces to Markovian in zero-delay limit.

Provides rules for feedback implementation using beam splitters.

Abstract

A quantum network is an open system consisting of several component Markovian input-output subsystems interconnected by boson field channels carrying quantum stochastic signals. Generalizing the work of Chebotarev and Gregoratti, we formulate the model description by prescribing a candidate Hamiltonian for the network including details the component systems, the field channels, their interconnections, interactions and any time delays arising from the geometry of the network. (We show that the candidate is a symmetric operator and proceed modulo the proof of self-adjointness.) The model is non-Markovian for finite time delays, but in the limit where these delays vanish we recover a Markov model and thereby deduce the rules for introducing feedback into arbitrary quantum networks. The type of feedback considered includes that mediated by the use of beam splitters. We are therefore able to give a system-theoretic approach to introducing connections between quantum mechanical state-based input-output systems, and give a unifying treatment using non-commutative fractional linear, or Mobius, transformations.